Computed Tomography scanning is a well known diagnostic technique and, in Its cone beam form, involves directing a wide beam of X-rays towards and through the patient and capturing the resulting two-dimensional image on a flat panel detector behind the patient. The apparatus (source and detector) is then rotated around the patient to obtain a multiplicity of images from different directions. These images are combined via a suitable computing means in order to produce a three-dimensional representation of the internal structure of the patient.
One limiting factor is the cost and size of the detector. Flat panel X-ray detectors are typically very expensive, and the cost increases with the dimensions of the detector. In practice this places an upper limit on the possible size of the flat panel detector. This in turn places a limit on the maximum aperture of the apparatus.
There are ways to increase the effective aperture of the device, within limits. Normally, the cone beam is directed along a central beam axis that coincides with the isocentre of the device, and the flat panel detector is centred on that beam axis. This will mean that each successive image taken by the flat panel detector will show a section of the patient centred on the isocentre. These can then be reconstructed in the normal way.
However, for particularly large patients this aperture may be insufficient. In this case, the aperture of the apparatus can be increased by moving the flat panel detector such that the central beam axis intersects near to one edge of the detector. This X-ray beam can then be collimated differently so that the cone beam is offset from the (previous) central beam axis and still covers the area of the flat panel detector. The beam will then be centred on an offset beam axis. In this case, each individual image will only show half of the relevant portion of the patient. However, after the apparatus has rotated through 180°, the other half will be brought into the image. When these images are reconstructed using a suitably reconstructed algorithm, a complete rendering of the patient will still be possible, albeit with a lower resolution reflecting the fact that each voxel of the reconstructed volume has been reconstructed using only half the amount of data.